Subject to section 410, an individual 18 years of age or over may be hospitalized or otherwise treated as a formal voluntary patient if either of the following applies:

  (a) The individual executes a written consent with the mental health facility for mental health treatment as a formal voluntary patient.

  (b) The full or limited guardian with authority to execute a written consent to mental health treatment, or a patient advocate authorized by the individual to make mental health treatment decisions under the estates and protected individuals code, 1998 PA 386, MCL 700.1101 to 700.8206, executes a written consent to provide mental health treatment.